1. Field of the Invention
The present invention generally relates to a sheet post-processing apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2009-67556 discloses an example of this type of sheet post-processing apparatus. The apparatus causes the leading edge of a sheet with an image formed thereon that is delivered to abut against a stopper member disposed in a feed path to accumulate the sheet in the feed path and determine a folding position of the sheet. The apparatus then brings a folding blade to abut against the folding position to fold the sheet. The sheet post-processing apparatus also includes a pair of folding rollers including a pair of roller members facing to each other and having a nip portion therebetween facing the edge of the folding blade across the sheet at the folding position. The folding blade is used to press the sheet, kept abutting against the stopper member, from the direction approximately perpendicular to the sheet surface, to cause the sheet to be folded in an L shape when viewed from the side. The sheet is then further pressed into the nip portion of the pair of folding rollers to be folded at a desired folding position.
To fold a plurality of stacked sheets, the sheets to be stacked and to be folded together are sequentially brought to abut against a stopper one after another, so that the sheets thus stacked are accumulated in the feed path. After accumulating a predetermined number of sheets in the feed path, a width aligning unit for aligning the sheet bundle in the width direction presses the sheet bundle on both ends thereof in the width direction to align the sheet bundle in the width direction. After aligning the sheet bundle in the width direction, the folding blade presses the sheet bundle into the nip portion of the pair of folding rollers to fold the bundle of the stacked sheets.
However, as the number of sheets accumulated in the feed path increases, the resistance between the sheets in the bundle also increases. Thus, it will be more difficult to move the sheets in the width direction upon aligning the width of the sheet bundle simply by pressing the sheet bundle on both ends thereof using the width aligning unit in the width direction. Therefore, the width of the sheet bundle may not be aligned precisely.
In Japanese Patent Application No. 2008-294436, the applicant of the present application suggested a sheet post-processing apparatus including a stacking disordering prevention mechanism, so that a plurality of sheets is stacked in the order in which the sheets are sent into the feed path upon conveying the sheets to abut against a stopper and to accumulate the sheets in the feed path. The stacking disordering prevention mechanism includes a pair of carriage rollers, and a pressing claw for pressing the trailing edge of accumulated sheets while covering it, at a position on the downstream side of the pair of carriage rollers in the sheet conveying direction. One end of the pressing claw is rotatably supported on a frame, and the other end thereof is displaceable between a position to block the feed path and another position escaped from the blocking position. A pulling coil spring keeps the other end of the pressing claw at the position to block the feed path. The biasing force of the pulling coil spring is set to a level by which the pressing claw is rotated in the opposite direction of the biasing direction of the pulling coil spring when the pressing claw is pressed by a sheet conveyed into the feed path, and escapes from the position to block the feed path so as to open the feed path. The sheet can reach the position of the stopper because the leading edge of the sheet conveyed into the feed path and conveyed by the pair of carriage rollers included in the stacking disordering prevention mechanism abuts against the pressing claw, pushes the pressing claw away, and moves toward the downstream side in the sheet conveying direction.
As the sheet pushes the pressing claw away and the trailing edge of the sheet passes through the pressing claw, the pulling coil spring moves the pressing claw to the position to block the feed path, and the pressing claw presses the trailing edge of the sheet in the manner covering it. When the second sheet is conveyed into the feed path, the pressing claw presses the trailing edge of the first sheet while covering it. Therefore, the second sheet passes through and is carried through the pressing claw in a manner overlapping the first sheet. The same operation is performed for the third and subsequent sheets to stack the sheets in the order in which the sheets are conveyed into the feed path.
Once the trailing edge of the sheet pushing the pressing claw away passes through the pair of carriage rollers included in the stacking disordering prevention mechanism, the sheet is carried toward the downstream side in the sheet conveying direction toward the stopper by its own weight. The trailing edge of the sheet, to which the power to go forward is no longer applied by the pair of carriage rollers, then fails to pass through the pressing claw only by its own weight, and the leading edge of the next sheet conveyed into the feed path may collide with the trailing edge to cause jamming.
Moreover, the sheet whose trailing edge has passed through the pair of carriage rollers in the stacking disordering prevention mechanism may be skewed or the leading edge of the sheet may fail to reach the stopper because the sheet is carried by its own weight. In response to this issue, a length aligning unit is optionally used for aligning a predetermined number of sheets accumulated in the feed path in the length direction (in the sheet conveying direction) by pushing the trailing edge of the sheet bundle, so that the leading edge of the sheets surely abuts against the stopper and the sheet bundle is aligned in the length direction. However, as the number of sheets accumulated in the feed path increases, the resistance between the sheets in the bundle also increases. Thus, it is more difficult to move the sheets in the length direction upon aligning the sheet bundle in the length direction simply by pressing the trailing edge the sheet bundle using the length aligning unit. Therefore, the sheet bundle may not be aligned precisely in the length direction.
In other words, when the sheet bundle is aligned in the width direction and the length direction after a predetermined number of sheets is accumulated in the feed path, the sheets may not be aligned precisely in the width direction and the length direction because the resistance between the sheets in the bundle increases as the number of sheets accumulated in the feed path increases. If the sheet bundle fails to be aligned precisely in the width direction and the length direction, the sheets may be folded at deviated folding positions while being stacked and folded.